Unconventional hydrocarbons production from low-permeability organic-rich shale and tight-sand formations is rapidly expanding. It opens vast new energy sources to the nation. Gas productions in 2012 from shale and tight-sand formations were, respectively, accounting for 34% and 24% of the nation's total gas production. Unconventional gas production is currently dominant in Marcellus Basin (29%), Haynesville Basin (23%), Barnett Basin (17%), and the remaining 31% is contributed by about 20 other basins. Future forecasts indicate that unconventional gas production will double by 2035 whereas unconventional oil production will increase by about 15%.
However, unconventional hydrocarbons exploration may impose adverse health and environmental long-term effects. In shale and tight-sand exploration, a fluid is used to fracture and stimulate the formation. This fluid is referred to as fracturing fluid or completion fluid. Fracturing fluid is typically potable water mixed with a large number of additives, and some of which are toxic chemicals. The average required volume of fracturing fluid is about 40,000 barrels for a vertical well and 100,000 barrels for a horizontal well.
The fluid that flows back during and after fracturing is often denoted flow-back water (FBW) or produced water (PW). However, it is referred to in this invention as PW. A portion of the fracturing fluid (e.g., 20-45%) flows back to the surface as PW, and the flow of PW substantially decreases with time to near halation at the well completion (30-90 days). During fracturing, ions including transition metals, scale-prone species, and Naturally Occurring Radioactive Materials (NORM) within formation layers are dissolved, mixed with high salinity formation water, and mobilized to the surface with PW. As such, ions concentrations in PW sharply increase with time; the longer the downhole residence time, the higher their concentrations in PW. Table 1 [Haluszczak, L. O., et al., Applied Geochemistry, 2013, 28, pp. 55-61] and Table 2 [Hayes, T. and Severin, B. F., RPSEA Final Report #08122-05, 2012] present, for example, some samples of PW at 10-14 days from, respectively, Marcellus and Barnett basins. Alkaline cations (magnesium, calcium, strontium, barium and radium), particularly radium radioactive isotopes, along with bromide are signatures for such PW.
Even though the 2005 Energy Policy Act exempts fracturing operations from the Safe Drinking Water Act (SDWA) with one exception (injection of diesel) to the exemption, hydro-fracturing is faced with surmounted predicaments driven by negative public perception. First, health and environmental regulations, monitoring, and enforcement for various contaminants by individual states are continually evolving, which are focused on the disclosure of the constituents in fracturing fluids and in PW for both discharge and reuse. One of the critical moves is the development of appropriate analytical methods, particularly for radioactivity, to replace the “drift” of using existing analytical procedures that may not be applicable to such PW and tend to underestimate the levels of some critical contaminants.
Second, the magnitude of PW is overwhelming. For example, PW from Marcellus basin was over 31 million barrels in 2014 alone. It indicates the least potable water dependency, if not overuse or depletion of potable water resources that may compete with other uses especially in water distressed areas.
Third, the toxic-nature of fracturing fluid as well as the possible toxic-radioactive nature of PW are alarming. The journey of fracturing fluid from the surface to downhole formation and back to the surface may contaminate shallow water aquifers via geological connectivity and leakage from fracturing fluid, produced hydrocarbons, PW, and PW holding ponds.
Fourth, the common method to avoid PW treatment is disposal in deep wells. However, the demand for disposal in deep wells has increased due to the high volume of PW. Thus, disposal in deep wells becomes very limited or may not be cheaply and readily available for producers in some states (e.g., Pennsylvania). In addition, the potential environmental risks of contaminating water aquifers (e.g., integrity failure of disposal wells, leakage from improper lining, etc.) as well as geological risks of potentially inducing seismicity and earthquakes are high. The costs of PW long distance hauling and disposal in deep wells therefore become relatively expensive for producers in some states. For example, the costs of trucking PW from Pennsylvania to a neighboring state and disposing of PW may be in the range of $10-$15 per barrel.
Fifth, the above mentioned factors open the door for promoting and developing innovative methods to properly treat and reuse PW. However, it appears that the predominant theme so far in treating PW is dilution, but dilution is not a solution to pollution. Incomplete treatment of PW; especially when the content of PW is released in both solid and liquid forms into landfills, possible public roads as a de-icing salt, conventional wastewater treatment facilities, surface waters; spreads contaminants wider and further even if the release of contaminants is within regulations. In establishing a proper PW treatment, no more logical approach seemed to present itself than the direct and selective isolation of radioactivity, scale prone species, and other critical species.